Fibre reinforced polymer components, otherwise known as polymer composite components, consist of reinforcing fibres held together with a polymer resin, often known as the matrix. This matrix can be a thermosetting polymer such as an epoxy resin, in which case the composite component can be called a thermoset composite component, or a thermoplastic polymer such as polyamide or polyetheretherketone, in which case the component can be called a thermoplastic composite component. It should be noted that a thermoset composite component may contain small amounts of thermoplastic polymer, for instance as a surfacing film, a resin additive, or a binder agent. A thermoplastic composite component may in the same way contain small amounts of thermoset polymer, for instance in a core or insert.
One of the best known thermoset composites is continuous carbon fibre reinforced epoxy (carbon-epoxy). This material is used to make much of the structure of the latest passenger aircraft. The material has many advantages, and the resulting components are strong, stiff, lightweight and fatigue resistant. However the components can be expensive to make and assemble. A key reason for the high cost of the assembly process is the difficulty of producing components with tight dimensional control on all surfaces, and of producing components with intricate surface shapes.
Most large structural thermoset composite components such as carbon-epoxy components are made using open (one-sided) moulds. A vacuum bag or other soft tooling surface is used to compact the reinforcing fibres and unreacted resin against the stiff mould. The whole assembly is then cured at elevated temperature. Due to variations in the thickness, or the weave structure of the reinforcements, or variations in the resin content, or variations in local temperature or pressure during the moulding process, the surface quality or texture of the “bag-side” surface of the resulting parts may vary substantially. More importantly, the local thickness of the part may vary significantly, leading to variation in the surface contour of the laminate, especially in the bag-side surface. This causes the manufacturer to either accept loose tolerances for the dimensions of the assembled structure, or to incur considerable difficulty and increased expense in the assembly of such composite laminates with other components. Massive and stiff assembly jigs may be necessary to hold the components in position while shimming is carried out to compensate for the local irregularities of the surface contour of the components.
A process which could be used to produce or shape structural thermoset composite components, made using open moulds, with accurate dimensions on any surface would be therefore be very desirable.
Shimming as described above can be carried out using solid sheets or laminates chosen to match the thickness of the gap to be filled. Another current method of shimming involves the use of “liquid shim”, which requires that the assembled components be held rigidly while the gap between them is filled with a thermosetting liquid resin that is then cured. In this case assembly requires a rigid and accurate tooling fixture to accurately place the components prior to curing of the liquid shim. The components then normally have to be removed and replaced prior to assembly, thus increasing the time and expense of the assembly operation. Therefore shimming using current techniques is not an ideal solution to the problem of thermoset composite components with loose dimensional tolerances.
A second related need, in the assembly of structural composite components, is for surface features to allow accurate location of adjacent components in joining operations. Such location points can be used to greatly simplify the process of assembly. Such features are also used for operations such as drilling and routing, in order to accurately locate the position of the component in machining operations. While it is possible to include location features such as depressions or steps in the surface of a cured thermoset composite component by building such features into the mould surface, this is generally not done due to the practical requirements of moulding, such as easy part removal, and the need for robust moulds with a long working life. The inclusion of some surface features in the mould surface may also detrimentally affect the structural properties of the resulting component, for example by causing local disruption of the reinforcing fibre paths. While it is often impractical, as explained above, to make thermoset composite components with location features on the mould surface, it is even more difficult or impractical to produce thermoset composite components with such location features on the bag side surface, due to the absence of a fixed or solid tooling surface against this surface of the curing component.
Another need is for a process to produce thermoset composite components with local surface features which cannot be made in a one-piece open mould. An example of these is a surface feature which would require negative “draft” in a normal open mould, that is, where the geometry of the required mould surface is such that the cured thermoset composite component would be impossible to remove from the mould without damage to the mould or the component. Such a feature might be desirable in order to provide a means of joining a second component through a snap-in joint. A similar need is for a process to enable a greater variety of external shapes in thermoset composite components made by a continuous production process such as pultrusion. The necessary mould shape normally used in such processes precludes the moulding of anything but the simplest-shaped thermoset composite component.
There is currently a process by which the surface of a thermoset composite component can be reshaped. This process requires additional uncured thermosetting resin or uncured thermosetting composite material to be placed against the surface of the component, shaped, and cured. This process requires that the additional material forms a good bond with the original surface, which can be difficult. This process also results in a thermoset composite component with additional mass, and the likelihood of an unsightly joint between the original component and the added material.
It is therefore desirable for the present invention to alleviate, at least in part, one or more of the above problems by providing a method for reshaping the surface of a thermoset composite component. Advantageously, the process may be used to prepare a working surface that enables the thermoset composite component to be easily joined with other components. More advantageously, the process is highly adaptable to a number of configurations and end uses, only some of which have been detailed above.